The present invention relates to a developing device included in a copier, facsimile apparatus, laser printer or similar electrophotographic image forming apparatus and a developing roller therefor.
Generally, a developing device included in an image forming apparatus of the kind described includes a developing roller, i.e., a magnet structure body having a magnet member fixed in place within a rotatable sleeve. The sleeve conveys a developer deposited thereon. Specifically, the developing roller is constituted by a metallic core, a plastic magnet or similar magnet member formed with a plurality of fixed magnetic poles, a rotatable sleeve formed of aluminum or similar nonmagnetic material, a drive flange, and a driven flange. When the developing roller is expected to operate with a magnetic carrier and nonmagnetic toner mixture, i.e., a two-ingredient type developer, it must be provided with a desired magnetic characteristic or flux density pattern.
A predominant procedure for producing the above magnet member molds a plastic magnet, rubber magnet or similar magnetic material by extrusion molding or injection molding while applying magnetic fields to the material (multipole orientation). The magnet member is, in many cases, implemented as a roll for the purpose of simplifying steps to follow the molding and enhancing efficient production. The roll type magnet member is a tubular resin body (referred to as a molding hereinafter). The molding may be produced by feeding resin containing a magnetic substance from an extruder to an orienting die in a tubular configuration and then applying magnetic fields for anisotropism (orientation),
In practice, the structure with the core and magnet member covering the core ts produced by molding the core and magnet member integrally or by inserting the core into the magnet member implemented as a pipe beforehand. The former scheme is rarely used because it is difficult to guarantee the angles of magnetic poles. The latter scheme forms a flat surface between magnetic poles and inserts the core by using the flat surface as a reference, as taught in Japanese Patent Laid-Open Publication No. 63-289908 by way of example. In any case, the problem with the conventional procedures is that it is difficult to provide the surface of the sleeve with a sophisticated magnetic characteristic including a repulsive magnetic pole.